Deflecting circuits



y 8, 12941- D. E. 'INORGAARDI 2,248,581 DEFLEC'I'ING CIRCUITS Filed July 10, 1940 FROM SOURCE or SYNCRONIZING DRIVING SOURCE POTENTIAL ANODE CURRENT ANODE VOLTAGE Ihverwtor: Donald E. Ncargaarwd,

His Attor ney.

Patented July 8, 1941 DEFLECTING CIRCUITS Donald E. Norgaard, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application July 10, 1940, Serial No. 344,717

Claims.

My invention relates to deflecting circuits for cathode ray apparatus. More specifically, it relates to a circuit employing a cathode ray discharge device having a magnetic deflecting element and it particularly contemplates improved means for adjusting the initial undeflected position of the cathode ray.

As is well known, means generally must be provided for causing the electron ray in a cathode ray discharge device to strike the screen or target at a predetermined point, usually the center, when the ray is in its normal undeflected position. Inaccuracies in manufacture or local influences, such as stray electrostatic or magnetic fields, otherwise may cause the entire figure traced by the beam to be offset in one direction or another when the device is energized, so that portions of the trace may not strike the screen or target.

With magnetic deflection the electron ray can be biased to an initial position by passing a direct current of proper magnitude and polarity through a desired deflecting coil. At the same time, it is desirable to maintain the alternating current load offered to the ray deflecting circuits substantially constant in order to avoid distortion of the wave form of the deflecting current in the coil, as will readily be appreciated by those skilled in the art.

Accordingly, it is a main object of my invention to provide improved means for adjusting the initial position of the electron ray in a cathode ray discharge device having current responsive deflecting means.

A further object of my invention is to provide means by which the cathode ray readily may be deflected magnetically in a desired direction and by any desired amount in order to adjust its initial position.

In accordance with my invention means are provided for passing a direct current through the magnetic deflecting element to bias the ray to the desired initial position. The magnitude and the direction of the direct current are under the control of a thermionic device in circuit with the ray deflecting element. The resistance of this device to direct current may be varied over a predetermined range, while at the same time the device offers a substantially constant, resistive, low alternating current impedance to deflecting currents flowing in the circuit.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 diagrammatically represents, partly in conventionalized form, one form of circuit embodying my invention, and Fig. 2 graphically illustrates certain characteristic curves which are helpful in understanding the operation of the circuit of Fig. 1.

A source of driving potential I!) is indicated in Fig. 1 in block form, since it may comprise any one of a number of well known circuits and the details thereof form no part of my invention. For example, it may comprise multi-vibrator or other sweep oscillator circuits for generating waves of a form suitable for excitation of the grid of an amplifying device [4 which, in turn, energizes the deflecting element of a cathode ray discharge device 28. These waves will generally be of complex wave form, comprising alternating current components of various frequencies, for causing an electron ray in the cathode ray device 26 to scan a target or screen therein in a predetermined manner. For example, the wave form of the potential supplied by the source It) may be as indicated graphically by the wave ll in Fig. 1, to provide linear deflection currents in the deflecting elements of the cathode ray device 26, as indicated graphically by the wave 42.

In television apparatus, in particular, it is often desired to synchronize the deflecting source with other circuits. This may be done in any well known manner by the injection of a related potential from a synchronizing source, not shown but indicated diagrammatically by the terminals H and [2 of Fig. 1.

The waves generated by the source it are applied through the coupling capacitor it upon the control grid of a thermionic amplifying device [4, which is illustrated as a pentode amplifier having an anode I5, suppressor grid l6, screen grid ll, control grid l8 and indirectly heated cathode l 9. A suitable negative bias potential is impressed upon the control grid [8 from a unidirectional potential source 20 through the coupling resistor 2i. Anode and screen grid potentials for the amplifier I4 are supplied from a suitable unidirectional potential source, indicated by the battery 22, which has a negative terminal thereof connected to the cathode of device 14 and a positive terminal thereof connected to the anode l5 and screen grid I! through the left-hand portion 23a of the resistor 23 and through the resistor 24, respectively. The source 22 preferably may be of the regulated type, such as is described in my application, Serial No. 264,751, filed March 29, 1939, now Patent No. 2,219,195, granted October 22, 1940, and assigned to the same assignee as the present invention, so that its impedance is negligible at all operating frequencies. The driving potentials applied to the control grid [8 of the device it control its anode current which is supplied to the deflection control element of the cathode ray discharge device 26. A bypass capacitor 25, large enough to prevent the potential of the screen grid i? of the device Hi from varying throughout a cycle of the driving wave an applied to control grid i8, is connected between screen grid i"! and cathode 19.

The device 26 is represented as a Well known form or" television picture signal generator having means therein for developing a cathode ray and projecting it against a sensitive mosaic target. The device also may include the usual means for controlling, focusing and accelerating the ray. However, the specific type of cathode ray device employed is immaterial to the practice of my invention and will not be described in detail. The device 26 might just as well comprise a cathode ray tube of one of the types commonly employed in oscillographic apparatus or television receiver circuits.

An element is illustrated in conjunction with the cathode ray device for magnetically deflecting the electron ray along one coordinate axis. This element is illustrated as a pair of magnetic deflecting coils 2?, though of course it may comprise only a single coil, or more than two coils. The coils also are shown connected in series, but they may optionally be connected in parallel. Suitable electrical networks known to the art, not shown, may also be associated with the coils 27 in order to improve the electrical characteristics thereof and to insure deflection of the cathode ray in a predetermined manner in accordance with currents supplied from the amplifier i l.

It will of course be understood that the oathode ray device 26 may be provided with other coordinate ray deflecting circuits which may be similar to the circuit illustrated, or of any other suitable types known to the art.

The deflecting currents 42 are supplied through the deflecting coils 27 in a circuit extending from the anode 55 through the conductor 28, the coils 21 in series, and the anode to cathode path of a thermionic control device 22 to the grounded cathode 59 of the device It.

The control device 29 is illustrated as a triode having an anode 30, control grid 3! and indirectly heated cathode 32. Anode potential is impressed upon the anode 39 from the source 22 through the right-hand portion 23b of the resistor 23. Also connected between the anode 30 and cathode 32 of device 29 is a circuit extending from anode 39, through a conductor 33, a capacitor 34, a resistor 35, and an adjustable tap 36 on a potentiometer 3'! to the grounded cathode 32. The control grid 3! of the device 29 is connected to a point on the resistor through a resistor 38 and an adjustable tap 39 on the resistor 35, for reasons that will shortly become apparent. A unidirectional potential source 49 is connected across the potentiometer 37 so that an adjustable negative bias may be applied to the grid 3! by adjusting the position of the tap 36.

It will now be observed that the deflecting coils 27 are connected across one diagonal of a Wheatstone bridge circuit as far as direct current is concerned. The portions 23a and 23b of the resistor 23 constitute two adjacent arms of the bridge, and the anode to cathode paths of the devices i l and 29 constitute the other two arms of the ridge. The potential source 22, which is connected across the other diagonal of the network, completes the bridge circuit. The resistances of the portions 23a and 23b are selected so as to be in an inverse ratio to the normal anode currents drawn by devices l4 and 29. These normal anode currents are, of course, determined by the potentials applied to their anodes rrom the source 22, by the negative bias potentials impressed on their grids from the sources 23 and 4.8 respectively, and by the potential of the screen grid ll of the device l-i. Preferably, the tap 3S occupies some intervening position on the potentiometer 3? under these conditions, so that the negative bias on grid 3! may either be increased or decreased from this initial value.

Under the above conditions the potentials of the anodes l5 and 39 will be the same and no direct current will flow through the deflecting coils. As will be explained in greater detail shortly, the anode to cathode potential of the control device 22 may be raised or lowered by adjusting the tap 36 on potentiometer 31. Thus, the bridge can be unbalanced in either direction and by any desired amount within limits sufficient to cause the electron ray in the cathode ray discharge device 26 to be deflected to any desired initial position.

As far as the alternating current circuit for the ray deflecting current waves is concerned, most of the anode current from device I4 flows from the anode through the coils 21 and through the anode to cathode path of the control device 29 to the cathode 19, as previously described, since the resistance of resistor 23 is relatively high compared to the impedance of the coils 21. Since a major portion of the deflecting current which flows through the coils 27 must also flow through the anode to cathode path of the device 29, it is desirable to provide means for lowering the impedance of this path to a small value in order to make eflicient use of the output current of the device l4. Also, as will be explained shortly, the anode to cathode impedance of the device 29 may also be made substantially resistive for ,these currents.

For greatest efficiency, the impedance offered by the control device 29 to deflecting currents must be small compared to the impedance of the coils 2'! in order that most of the potential drop may occur across the useful portion of the circuit. If the device 29 were replaced by a variable ohmic resistance in order to control the distribution of direct current in the bridge network, it would necessarily have to be of relatively high resistance to match the anode to cathode resistance of the amplifier I4. It would therefore ofier the same high impedance to deflecting currents. Of course, the impedance to deflecting currents could be reduced by shunting a capacitor thereacross but in such case, unless a very large capacitor were used, undesirable phase shift between the various alternating current components of the complex deflecting waves would unavoidably be produced, causing wave form distortion. It is true that such distortion might be compensated by suitable networks known to the art, associated with the coils 21 or the source I0; however, in either case, any readjustment of the resistor, in order to alter the undefiected position of the ray in device 26, would alter these conditions and would require a simultaneous readjustment of the corrective network.

The control device 29 and its associated cir: cuits avoid all these difficulties which would be introduced if it were replaced by an ohmic resistance element or a resistance element shunted by a capacitance element. As will now be explained more in detail, the device 29 may be adjusted to present a substantially resistive and constant low impedance to alternating currents, while at the same time its resistance to direct current may be varied Within limits adequate to secure adjustment of the initial position of the electron ray within the device 26.

In accordance with my invention the impedance of the capacitor 34 is made Very low as compared to the impedance of the resistor 35 for all frequency components down to the lowest frequency component present in the deflecting current waves. Thus, a low impedance alternating current connection is provided between the anode 30 and the grid 3|.

If the tap 39 is assumed to be positioned at the junction of capacitor 34 and resistor 35 for the present, then the alternating potential upon the grid 3| is substantially equal to and in phase with the alternating potential upon the anode 30. It

is well known in vacuum tube theory that the instantaneous anode current in a grid control discharge device, under certain conditions, can be expressed as where i is the instantaneous value of the alternating current component of anode current, Rp is the anode resistance, 6p is the instantaneous value of the alternating current component of anode potential, ,u. is the amplification factor and 6g is the instantaneous value of the alternating current component of grid potential. The above relation holds true provided that the fluctuation in the anode current takes place on the linear portion of the anode current-grid potential characteristic curve. In other words, Equation 1 applies exactly when the anode current flows continuously and varies linearly with grid potential, 1. e. when operation is on a linear portion of the class A range. Since the .control device 29 neither draws grid current nor approaches anode current cutoff, these requirements are substantially met.

As previously mentioned, since the impedance of capacitor 34 is low, under the assumed conditions:

a e a: Substituting in Equation 1, it follows that Lp= R i l Thus, the anode to cathode impedance Z of the device 29 for alternating currents is Since vacuum tube constants are often supplied in terms of the mutual conductance Gin,

and anode resistance, Rp, it may be more conven- By choosing a thermionic amplifier of high mutual conductance for the device 29 a considerable variation in direct current resistance can be secured with a relatively small change in grid bias, as determined by the potentiometer 37. At the same time it will be observed from examination of Equation 4 or 5 that the alternating current impedance is not aifecteol. Furthermore, this impedance will in general be low compared to the impedance of the magnetic deflecting element so that the effect of small variations in the alternating current impedance of device 29 due to normal curvature of the tube characteristics is negligible.

Possibly the function of control device 29 may be more easily seen from Fig. 2 which represents anode current-anode voltage curves which are characteristic of one form of high-Gm triode that may be employed successfully. The curve 50 represents the variation in anode current with anode voltage under static conditions and with zero grid bias. The other curves 53a, 5%, etc., to the right represent similar variations with progressively increasing negative grid bias potentials.

Assuming that a particular operating point 5|, for operation on a linear portion of the class A range, is selected, it can be determined easily from Equation 4 or 5 that, under alternating current conditions, the instantaneous anode voltage and instantaneous anode current will vary in a manner similar to that illustrated by the load line 53, for example.

It is apparent that the slope of the load line 53 is steep, and the impedance of the device 29 is therefore low for alternating currents. Since the static characteristic curves also have steep slopes, it will be seen that a small change in grid bias will produce a considerable shift in the operating point 5!. At the same time, if class A conditions are maintained, the alternating current impedance, i. e., the slope of the load line, will not be varied appreciably. Furthermore, the device 29 usually need be operated over only a relatively small range for the purposes of my invention and it is very simple to maintain substantially linear operating conditions.

An ohmic resistance employed in place of the device 29 would of course have a ratio of voltage divided by current times resistance equal to unity, as indicated graphically by the dotted line 54 in Fig. 2. The alternating current impedance would be many times as great as that of the device 29 as is evident from comparing the slopes of the lines 53 and 54.

As a further refinement, the alternating current impedance of device 29 may be varied, if desired, by moving the tap 39 on the resistor 35. Moving the tap 39 downwardly produces no change in phase of the grid voltage as compared with the anode voltage but merely reduces the proportion of the anode voltage applied to the grid, proportionately increasing the alternating current impedance of the device. Ordinarily, however, it is preferable to make the alternating current impedance as low as possible, so that the grid 3! may well be connected permanently to the junction of capacitor 34 and resistor 35.

The resistor 38 is of low resistance and is included in the grid circuit of the device 29 mere- 1y to preclude the possibility of parasitic oscillations occurring therein as a result of the feedback coupling between anode and grid.

As an exemplification of particular values of circuit constants which I have found to be suitable in a particular television transmitter apparatus embodying my invention, the following constants are given, merely by way of illustration and not by way of limitation. Thus, in this particular apparatus, the source it provided deflecting currents of complex wave form having a fundamental 60 cycle component. The exact type of amplifier chosen for the device M is not material. A type 6L6 or type 6V6 beam power tetrode, rather than a pentode as shown, has been found to give satisfactory results. The impedance of the coils 27 in series was about 1000 ohms, being almost pure resistance in this particular case. For the control device 29 a type 2A3 high-Gm power amplifier triode was selected. This type of amplifier has a mutual conductance, Gm, of approximately 5350 micromhos, an anode resistance, R of about 800 ohms, and consequently an amplification factor, of about 4.2. From Equation 4 or it follows that the anode to cathode impedance to alternating currents is approximately 154 ohms, -a value 'sufficiently low compared to the impedance of coils 27 so that about 85 per cent of the total alternating current potential across coils 2'! and amplifier 2 9 can appear on coils 2?, The capacitor 34 had a capacitance of one microfarad and the resistor 35 a resistanceoi 100,000 ohms. Thus, the impedance of capacitor 34 was less than three per cent of the impedance of resistor 35 at 60 cycles, introducing negligible phase shift between the instantaneous potentials of anode 3t and grid 3 I Thus, it will be appreciated that I have provided a very effective means for controlling the direction and magnitude of direct current through the deflecting element of a cathode ray discharge device, while at the same time the normal operation of the device is not appreciably affected and no distortion of the wave form of the deflecting currents is introduced. Vfhile it is in no wise lim- 'ted thereto, it will be apparent that my invention is particularly useful in applications where the fundamental sweep frequency is of a relatively low value.

While I have shown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since various modification-s may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination with a cathode ray device having a magnetic deflecting coil, a source of deflecting currents, an electron discharge device, means to connect said coil, said source and said discharge device in series, a unidirectional potential source, means to connect said unidirectional source across said coil and said device, means to give to said discharge device a substantially constant, low, alternating current impedance to deflecting currents and a predetermined resistance to direct current, and means tovary said resistance.

2. In combination, a current responsive ray deflecting means, a source of deflecting currents, an electron discharge device having anode, cathode and grid electrodes, means to couple said source to said deflecting means through the anode to cathode path of said device, means to impress a unidirectional potential upon said deflecting means and said path, means to give to said device a substantially constant, low, alternating current impedance to said deflecting currents and a predetermined resistance to direct current, said last means including means to impress a predetermined bias potential between said grid and said cathode, and means to vary said bias potential, whereby the resistance of said path to direct current may be varied.

3. The combination, in a system for centering the cathode ray in a cathode ray devicehaving a magnetic deflecting coil, of a source of deflecting currents, an electron discharge device having an anode, a cathode and a grid, means serially interconnecting said source, said coil and the anode to cathode path of said discharge device, means to impress a unidirectional potential upon said coil and said path, means to give to said device a low alternating current impedance to deflecting currents and a predetermined resistance to direct current, said means comprising a capacitor included between said anode and grid and a resistor included between said grid and cathode, means to bias said grid negatively with respect to said cathode, and means to vary said bias, whereby the resistance of said path may be varied substantially independently of its impedance thereby to vary the direct current in said coil for centering said ray.

4. In a ray deflecting circuit, the combination with a current responsive ray deflecting element and a source of deflecting current waves having alternating current components of a plurality of different frequencies, of a thermionic amplifier having anode, cathode and grid electrodes, means serially interconnecting said source, said deflecting element and the anode to cathode path of said amplifier, means to impress a unidirectional potential upon said element and said path, a capacitor of relatively low impedance to the lowest frequency component of said currents connected between said grid and said element, means to impress a predetermined bias potential between said grid and said cathode through a direct current connection of relatively high impedance to all components of said currents, and means to vary the magnitude of said bias potential, whereby the resistance of said path to direct current may be varied.

5. In a system for centering the cathode ray in a cathode ray discharge device, the combination of a magnetic deflecting coil for said device, a source of deflecting currents, a thermionic amplifler having an anode, a cathode and a grid, means serially interconnecting said source, said coil and the anode to cathode path of said amplifier, means to impress anode potential for said amplifier across said coil and said path, means to impress on said grid an alternating potential substantially in phase with and proportional to the deflecting currents in said coil, whereby said amplifier offers a substantially constant, low, alternating current impedance to said currents, means to bias said grid for operation of said amplifier on a substantially linear portion of its anode current-grid potential characteristic, and means to vary said bias, thereby to vary the direct current drawn by said amplifier through said coil for centering the ray.

6. The combination, in a system for centering the figure produced by the cathode ray on the screen of a cathode ray device, of a deflecting coil for said device, a pair of electron discharge devices, said discharge devices having their cathodes connected together and their respective anodes connected to opposite terminals of said coil, means to supply anode operating potential to said devices, control electrodes in said devices, means to supply deflecting potentials to the control electrode of one of said devices to control the deflection of said ray to produce a desired figure on said screen, and means to adjust the potential of the control electrode of the other discharge device to center said figure upon said screen.

7. In a system for centering the figure produced by the cathode ray on the screen of a cathode ray device, the combination of a deflecting coil for said device, a pair of electron discharge devices, said discharge devices having the cathodes connected together and their respective anodes connected to opposite terminals of said coil, means to supply anode operating potential to said devices, control grids in said devices, means to impress deflecting potentials on the grid of one of said devices to control the ray deflecting currents in said coil, means to impress on the grid of the other discharge'device alternating potentials substantially in phase with and proportional to said deflecting currents in said coil, thereby to give said other device a substantially constant, low impedance to said currents, means to bias said grid negatively, and means to vary said bias, whereby the anode to cathode resistance of said other device may be varied thereby to vary the direct current in said coil for centering said ray upon said screen.

8. In a system for centering the cathode ray on the screen of a cathode ray device, the combination of a Wheatstone bridge circuit, a deflecting coil for said device connected across one diagonal of said bridge, a source of unidirectional potential connected across the opposite diagonal of said bridge, electron discharge devices in the two arms of said bridge adjacent one terminal of said source, control electrodes in said devices, means to supply deflecting potentials to the control electrode of one of said devices to control deflecting currents in said coil, and means to adjust the potential of the control electrode of the other discharge device to vary the direction and magnitude of direct current in said coil for centering said ray.

9. In a system for centering the cathode ray on the screen of a cathode ray device, the combination of a Wheatstone bridge circuit comprising a pair of electron discharge devices and two adjacent arms of the bridge with their cathodes connected to one corner and a pair of impedances in the other two arms with their junctions at the diagonally opposite corner, a source of anode potential for said discharge devices connected across said corners, a deflecting coil for said cathode ray device connected across the other diagonal of said bridge, control grids in said discharge devices, means to supply deflecting potentials to the grid of one of said devices to control deflecting currents in said coil, means to give said other discharge device a constant, low, alternating current impedance to deflecting currents and a predetermined resistance to direct current, and means to vary said resistance comprising means to vary the bias on the grid of said other device, whereby the direction and magnitude of direct current in said coil may be varied for centering said ray on the screen.

10. In a system for centering the cathode ray on the screen of a cathode ray device, the combination of a Wheatstone bridge circuit comprising a pair of electron discharge devices in two adjacent arms of the bridge, with their cathodes connected to one corner and a pair of impedances in the other two arms with their junctions at the diagonally opposite corner, a source of anode potential for said discharge devices connected across said corners, a deflecting coil for said cathode ray device connected across the other diagonal of said bridge, control grids in said discharge devices, means to supply deflecting potentials to the grid of one of said devices to control deflecting currents in said coil, means to impress on the grid of the other device alternating potentials substantially in phase with and proportional to the deflecting currents in said coil, means to bias said grid negatively, and means to vary said bias whereby the anode resistance of said other device may be varied to control the direction and magnitude in said coil for centering said ray on the screen.

DONALD E. NORGAARD. 

